/*
 * jquant2.c
 *
 * Copyright (C) 1991-1995, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains 2-pass color quantization (color mapping) routines.
 * These routines provide selection of a custom color map for an image,
 * followed by mapping of the image to that color map, with optional
 * Floyd-Steinberg dithering.
 * It is also possible to use just the second pass to map to an arbitrary
 * externally-given color map.
 *
 * Note: ordered dithering is not supported, since there isn't any fast
 * way to compute intercolor distances; it's unclear that ordered dither's
 * fundamental assumptions even hold with an irregularly spaced color map.
 */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"

#ifdef QUANT_2PASS_SUPPORTED


/*
 * This module implements the well-known Heckbert paradigm for color
 * quantization.  Most of the ideas used here can be traced back to
 * Heckbert's seminal paper
 *   Heckbert, Paul.  "Color Image Quantization for Frame Buffer Display",
 *   Proc. SIGGRAPH '82, Computer Graphics v.16 #3 (July 1982), pp 297-304.
 *
 * In the first pass over the image, we accumulate a histogram showing the
 * usage count of each possible color.  To keep the histogram to a reasonable
 * size, we reduce the precision of the input; typical practice is to retain
 * 5 or 6 bits per color, so that 8 or 4 different input values are counted
 * in the same histogram cell.
 *
 * Next, the color-selection step begins with a box representing the whole
 * color space, and repeatedly splits the "largest" remaining box until we
 * have as many boxes as desired colors.  Then the mean color in each
 * remaining box becomes one of the possible output colors.
 * 
 * The second pass over the image maps each input pixel to the closest output
 * color (optionally after applying a Floyd-Steinberg dithering correction).
 * This mapping is logically trivial, but making it go fast enough requires
 * considerable care.
 *
 * Heckbert-style quantizers vary a good deal in their policies for choosing
 * the "largest" box and deciding where to cut it.  The particular policies
 * used here have proved out well in experimental comparisons, but better ones
 * may yet be found.
 *
 * In earlier versions of the IJG code, this module quantized in YCbCr color
 * space, processing the raw upsampled data without a color conversion step.
 * This allowed the color conversion math to be done only once per colormap
 * entry, not once per pixel.  However, that optimization precluded other
 * useful optimizations (such as merging color conversion with upsampling)
 * and it also interfered with desired capabilities such as quantizing to an
 * externally-supplied colormap.  We have therefore abandoned that approach.
 * The present code works in the post-conversion color space, typically RGB.
 *
 * To improve the visual quality of the results, we actually work in scaled
 * RGB space, giving G distances more weight than R, and R in turn more than
 * B.  To do everything in integer math, we must use integer scale factors.
 * The 2/3/1 scale factors used here correspond loosely to the relative
 * weights of the colors in the NTSC grayscale equation.
 * If you want to use this code to quantize a non-RGB color space, you'll
 * probably need to change these scale factors.
 */

#define R_SCALE 2				/* scale R distances by this much */
#define G_SCALE 3				/* scale G distances by this much */
#define B_SCALE 1				/* and B by this much */

/* Relabel R/G/B as components 0/1/2, respecting the RGB ordering defined
 * in jmorecfg.h.  As the code stands, it will do the right thing for R,G,B
 * and B,G,R orders.  If you define some other weird order in jmorecfg.h,
 * you'll get compile errors until you extend this logic.  In that case
 * you'll probably want to tweak the histogram sizes too.
 */

#if RGB_RED == 0
#define C0_SCALE R_SCALE
#endif
#if RGB_BLUE == 0
#define C0_SCALE B_SCALE
#endif
#if RGB_GREEN == 1
#define C1_SCALE G_SCALE
#endif
#if RGB_RED == 2
#define C2_SCALE R_SCALE
#endif
#if RGB_BLUE == 2
#define C2_SCALE B_SCALE
#endif


/*
 * First we have the histogram data structure and routines for creating it.
 *
 * The number of bits of precision can be adjusted by changing these symbols.
 * We recommend keeping 6 bits for G and 5 each for R and B.
 * If you have plenty of memory and cycles, 6 bits all around gives marginally
 * better results; if you are short of memory, 5 bits all around will save
 * some space but degrade the results.
 * To maintain a fully accurate histogram, we'd need to allocate a "long"
 * (preferably unsigned long) for each cell.  In practice this is overkill;
 * we can get by with 16 bits per cell.  Few of the cell counts will overflow,
 * and clamping those that do overflow to the maximum value will give close-
 * enough results.  This reduces the recommended histogram size from 256Kb
 * to 128Kb, which is a useful savings on PC-class machines.
 * (In the second pass the histogram space is re-used for pixel mapping data;
 * in that capacity, each cell must be able to store zero to the number of
 * desired colors.  16 bits/cell is plenty for that too.)
 * Since the JPEG code is intended to run in small memory model on 80x86
 * machines, we can't just allocate the histogram in one chunk.  Instead
 * of a true 3-D array, we use a row of pointers to 2-D arrays.  Each
 * pointer corresponds to a C0 value (typically 2^5 = 32 pointers) and
 * each 2-D array has 2^6*2^5 = 2048 or 2^6*2^6 = 4096 entries.  Note that
 * on 80x86 machines, the pointer row is in near memory but the actual
 * arrays are in far memory (same arrangement as we use for image arrays).
 */

#define MAXNUMCOLORS  (MAXJSAMPLE+1)	/* maximum size of colormap */

/* These will do the right thing for either R,G,B or B,G,R color order,
 * but you may not like the results for other color orders.
 */
#define HIST_C0_BITS  5			/* bits of precision in R/B histogram */
#define HIST_C1_BITS  6			/* bits of precision in G histogram */
#define HIST_C2_BITS  5			/* bits of precision in B/R histogram */

/* Number of elements along histogram axes. */
#define HIST_C0_ELEMS  (1<<HIST_C0_BITS)
#define HIST_C1_ELEMS  (1<<HIST_C1_BITS)
#define HIST_C2_ELEMS  (1<<HIST_C2_BITS)

/* These are the amounts to shift an input value to get a histogram index. */
#define C0_SHIFT  (BITS_IN_JSAMPLE-HIST_C0_BITS)
#define C1_SHIFT  (BITS_IN_JSAMPLE-HIST_C1_BITS)
#define C2_SHIFT  (BITS_IN_JSAMPLE-HIST_C2_BITS)


typedef UINT16  histcell;		/* histogram cell; prefer an unsigned type */

typedef histcell FAR *histptr;	/* for pointers to histogram cells */

typedef histcell hist1d[HIST_C2_ELEMS];	/* typedefs for the array */
typedef hist1d FAR *hist2d;		/* type for the 2nd-level pointers */
typedef hist2d *hist3d;			/* type for top-level pointer */


/* Declarations for Floyd-Steinberg dithering.
 *
 * Errors are accumulated into the array fserrors[], at a resolution of
 * 1/16th of a pixel count.  The error at a given pixel is propagated
 * to its not-yet-processed neighbors using the standard F-S fractions,
 *		...	(here)	7/16
 *		3/16	5/16	1/16
 * We work left-to-right on even rows, right-to-left on odd rows.
 *
 * We can get away with a single array (holding one row's worth of errors)
 * by using it to store the current row's errors at pixel columns not yet
 * processed, but the next row's errors at columns already processed.  We
 * need only a few extra variables to hold the errors immediately around the
 * current column.  (If we are lucky, those variables are in registers, but
 * even if not, they're probably cheaper to access than array elements are.)
 *
 * The fserrors[] array has (#columns + 2) entries; the extra entry at
 * each end saves us from special-casing the first and last pixels.
 * Each entry is three values long, one value for each color component.
 *
 * Note: on a wide image, we might not have enough room in a PC's near data
 * segment to hold the error array; so it is allocated with alloc_large.
 */

#if BITS_IN_JSAMPLE == 8
typedef INT16   FSERROR;		/* 16 bits should be enough */
typedef int     LOCFSERROR;		/* use 'int' for calculation temps */
#else
typedef INT32   FSERROR;		/* may need more than 16 bits */
typedef INT32   LOCFSERROR;		/* be sure calculation temps are big enough */
#endif

typedef FSERROR FAR *FSERRPTR;	/* pointer to error array (in FAR storage!) */


/* Private subobject */

typedef struct
{
	struct jpeg_color_quantizer pub;	/* public fields */

	/* Space for the eventually created colormap is stashed here */
	JSAMPARRAY      sv_colormap;	/* colormap allocated at init time */
	int             desired;	/* desired # of colors = size of colormap */

	/* Variables for accumulating image statistics */
	hist3d          histogram;	/* pointer to the histogram */

	boolean         needs_zeroed;	/* TRUE if next pass must zero histogram */

	/* Variables for Floyd-Steinberg dithering */
	FSERRPTR        fserrors;	/* accumulated errors */
	boolean         on_odd_row;	/* flag to remember which row we are on */
	int            *error_limiter;	/* table for clamping the applied error */
} my_cquantizer;

typedef my_cquantizer *my_cquantize_ptr;


/*
 * Prescan some rows of pixels.
 * In this module the prescan simply updates the histogram, which has been
 * initialized to zeroes by start_pass.
 * An output_buf parameter is required by the method signature, but no data
 * is actually output (in fact the buffer controller is probably passing a
 * NULL pointer).
 */

METHODDEF void prescan_quantize(j_decompress_ptr cinfo, JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	register JSAMPROW ptr;
	register histptr histp;
	register hist3d histogram = cquantize->histogram;
	int             row;
	JDIMENSION      col;
	JDIMENSION      width = cinfo->output_width;

	for(row = 0; row < num_rows; row++)
	{
		ptr = input_buf[row];
		for(col = width; col > 0; col--)
		{
			/* get pixel value and index into the histogram */
			histp = &histogram[GETJSAMPLE(ptr[0]) >> C0_SHIFT][GETJSAMPLE(ptr[1]) >> C1_SHIFT][GETJSAMPLE(ptr[2]) >> C2_SHIFT];
			/* increment, check for overflow and undo increment if so. */
			if(++(*histp) <= 0)
				(*histp)--;
			ptr += 3;
		}
	}
}


/*
 * Next we have the really interesting routines: selection of a colormap
 * given the completed histogram.
 * These routines work with a list of "boxes", each representing a rectangular
 * subset of the input color space (to histogram precision).
 */

typedef struct
{
	/* The bounds of the box (inclusive); expressed as histogram indexes */
	int             c0min, c0max;
	int             c1min, c1max;
	int             c2min, c2max;
	/* The volume (actually 2-norm) of the box */
	INT32           volume;
	/* The number of nonzero histogram cells within this box */
	long            colorcount;
} box;

typedef box    *boxptr;


LOCAL           boxptr find_biggest_color_pop(boxptr boxlist, int numboxes)
/* Find the splittable box with the largest color population */
/* Returns NULL if no splittable boxes remain */
{
	register boxptr boxp;
	register int    i;
	register long   maxc = 0;
	boxptr          which = NULL;

	for(i = 0, boxp = boxlist; i < numboxes; i++, boxp++)
	{
		if(boxp->colorcount > maxc && boxp->volume > 0)
		{
			which = boxp;
			maxc = boxp->colorcount;
		}
	}
	return which;
}


LOCAL           boxptr find_biggest_volume(boxptr boxlist, int numboxes)
/* Find the splittable box with the largest (scaled) volume */
/* Returns NULL if no splittable boxes remain */
{
	register boxptr boxp;
	register int    i;
	register INT32  maxv = 0;
	boxptr          which = NULL;

	for(i = 0, boxp = boxlist; i < numboxes; i++, boxp++)
	{
		if(boxp->volume > maxv)
		{
			which = boxp;
			maxv = boxp->volume;
		}
	}
	return which;
}


LOCAL void update_box(j_decompress_ptr cinfo, boxptr boxp)
/* Shrink the min/max bounds of a box to enclose only nonzero elements, */
/* and recompute its volume and population */
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	hist3d          histogram = cquantize->histogram;
	histptr         histp;
	int             c0, c1, c2;
	int             c0min, c0max, c1min, c1max, c2min, c2max;
	INT32           dist0, dist1, dist2;
	long            ccount;

	c0min = boxp->c0min;
	c0max = boxp->c0max;
	c1min = boxp->c1min;
	c1max = boxp->c1max;
	c2min = boxp->c2min;
	c2max = boxp->c2max;

	if(c0max > c0min)
		for(c0 = c0min; c0 <= c0max; c0++)
			for(c1 = c1min; c1 <= c1max; c1++)
			{
				histp = &histogram[c0][c1][c2min];
				for(c2 = c2min; c2 <= c2max; c2++)
					if(*histp++ != 0)
					{
						boxp->c0min = c0min = c0;
						goto have_c0min;
					}
			}
  have_c0min:
	if(c0max > c0min)
		for(c0 = c0max; c0 >= c0min; c0--)
			for(c1 = c1min; c1 <= c1max; c1++)
			{
				histp = &histogram[c0][c1][c2min];
				for(c2 = c2min; c2 <= c2max; c2++)
					if(*histp++ != 0)
					{
						boxp->c0max = c0max = c0;
						goto have_c0max;
					}
			}
  have_c0max:
	if(c1max > c1min)
		for(c1 = c1min; c1 <= c1max; c1++)
			for(c0 = c0min; c0 <= c0max; c0++)
			{
				histp = &histogram[c0][c1][c2min];
				for(c2 = c2min; c2 <= c2max; c2++)
					if(*histp++ != 0)
					{
						boxp->c1min = c1min = c1;
						goto have_c1min;
					}
			}
  have_c1min:
	if(c1max > c1min)
		for(c1 = c1max; c1 >= c1min; c1--)
			for(c0 = c0min; c0 <= c0max; c0++)
			{
				histp = &histogram[c0][c1][c2min];
				for(c2 = c2min; c2 <= c2max; c2++)
					if(*histp++ != 0)
					{
						boxp->c1max = c1max = c1;
						goto have_c1max;
					}
			}
  have_c1max:
	if(c2max > c2min)
		for(c2 = c2min; c2 <= c2max; c2++)
			for(c0 = c0min; c0 <= c0max; c0++)
			{
				histp = &histogram[c0][c1min][c2];
				for(c1 = c1min; c1 <= c1max; c1++, histp += HIST_C2_ELEMS)
					if(*histp != 0)
					{
						boxp->c2min = c2min = c2;
						goto have_c2min;
					}
			}
  have_c2min:
	if(c2max > c2min)
		for(c2 = c2max; c2 >= c2min; c2--)
			for(c0 = c0min; c0 <= c0max; c0++)
			{
				histp = &histogram[c0][c1min][c2];
				for(c1 = c1min; c1 <= c1max; c1++, histp += HIST_C2_ELEMS)
					if(*histp != 0)
					{
						boxp->c2max = c2max = c2;
						goto have_c2max;
					}
			}
  have_c2max:

	/* Update box volume.
	 * We use 2-norm rather than real volume here; this biases the method
	 * against making long narrow boxes, and it has the side benefit that
	 * a box is splittable iff norm > 0.
	 * Since the differences are expressed in histogram-cell units,
	 * we have to shift back to JSAMPLE units to get consistent distances;
	 * after which, we scale according to the selected distance scale factors.
	 */
	dist0 = ((c0max - c0min) << C0_SHIFT) * C0_SCALE;
	dist1 = ((c1max - c1min) << C1_SHIFT) * C1_SCALE;
	dist2 = ((c2max - c2min) << C2_SHIFT) * C2_SCALE;
	boxp->volume = dist0 * dist0 + dist1 * dist1 + dist2 * dist2;

	/* Now scan remaining volume of box and compute population */
	ccount = 0;
	for(c0 = c0min; c0 <= c0max; c0++)
		for(c1 = c1min; c1 <= c1max; c1++)
		{
			histp = &histogram[c0][c1][c2min];
			for(c2 = c2min; c2 <= c2max; c2++, histp++)
				if(*histp != 0)
				{
					ccount++;
				}
		}
	boxp->colorcount = ccount;
}


LOCAL int median_cut(j_decompress_ptr cinfo, boxptr boxlist, int numboxes, int desired_colors)
/* Repeatedly select and split the largest box until we have enough boxes */
{
	int             n, lb;
	int             c0, c1, c2, cmax;
	register boxptr b1, b2;

	while(numboxes < desired_colors)
	{
		/* Select box to split.
		 * Current algorithm: by population for first half, then by volume.
		 */
		if(numboxes * 2 <= desired_colors)
		{
			b1 = find_biggest_color_pop(boxlist, numboxes);
		}
		else
		{
			b1 = find_biggest_volume(boxlist, numboxes);
		}
		if(b1 == NULL)			/* no splittable boxes left! */
			break;
		b2 = &boxlist[numboxes];	/* where new box will go */
		/* Copy the color bounds to the new box. */
		b2->c0max = b1->c0max;
		b2->c1max = b1->c1max;
		b2->c2max = b1->c2max;
		b2->c0min = b1->c0min;
		b2->c1min = b1->c1min;
		b2->c2min = b1->c2min;
		/* Choose which axis to split the box on.
		 * Current algorithm: longest scaled axis.
		 * See notes in update_box about scaling distances.
		 */
		c0 = ((b1->c0max - b1->c0min) << C0_SHIFT) * C0_SCALE;
		c1 = ((b1->c1max - b1->c1min) << C1_SHIFT) * C1_SCALE;
		c2 = ((b1->c2max - b1->c2min) << C2_SHIFT) * C2_SCALE;
		/* We want to break any ties in favor of green, then red, blue last.
		 * This code does the right thing for R,G,B or B,G,R color orders only.
		 */
#if RGB_RED == 0
		cmax = c1;
		n = 1;
		if(c0 > cmax)
		{
			cmax = c0;
			n = 0;
		}
		if(c2 > cmax)
		{
			n = 2;
		}
#else
		cmax = c1;
		n = 1;
		if(c2 > cmax)
		{
			cmax = c2;
			n = 2;
		}
		if(c0 > cmax)
		{
			n = 0;
		}
#endif
		/* Choose split point along selected axis, and update box bounds.
		 * Current algorithm: split at halfway point.
		 * (Since the box has been shrunk to minimum volume,
		 * any split will produce two nonempty subboxes.)
		 * Note that lb value is max for lower box, so must be < old max.
		 */
		switch (n)
		{
			case 0:
				lb = (b1->c0max + b1->c0min) / 2;
				b1->c0max = lb;
				b2->c0min = lb + 1;
				break;
			case 1:
				lb = (b1->c1max + b1->c1min) / 2;
				b1->c1max = lb;
				b2->c1min = lb + 1;
				break;
			case 2:
				lb = (b1->c2max + b1->c2min) / 2;
				b1->c2max = lb;
				b2->c2min = lb + 1;
				break;
		}
		/* Update stats for boxes */
		update_box(cinfo, b1);
		update_box(cinfo, b2);
		numboxes++;
	}
	return numboxes;
}


LOCAL void compute_color(j_decompress_ptr cinfo, boxptr boxp, int icolor)
/* Compute representative color for a box, put it in colormap[icolor] */
{
	/* Current algorithm: mean weighted by pixels (not colors) */
	/* Note it is important to get the rounding correct! */
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	hist3d          histogram = cquantize->histogram;
	histptr         histp;
	int             c0, c1, c2;
	int             c0min, c0max, c1min, c1max, c2min, c2max;
	long            count;
	long            total = 0;
	long            c0total = 0;
	long            c1total = 0;
	long            c2total = 0;

	c0min = boxp->c0min;
	c0max = boxp->c0max;
	c1min = boxp->c1min;
	c1max = boxp->c1max;
	c2min = boxp->c2min;
	c2max = boxp->c2max;

	for(c0 = c0min; c0 <= c0max; c0++)
		for(c1 = c1min; c1 <= c1max; c1++)
		{
			histp = &histogram[c0][c1][c2min];
			for(c2 = c2min; c2 <= c2max; c2++)
			{
				if((count = *histp++) != 0)
				{
					total += count;
					c0total += ((c0 << C0_SHIFT) + ((1 << C0_SHIFT) >> 1)) * count;
					c1total += ((c1 << C1_SHIFT) + ((1 << C1_SHIFT) >> 1)) * count;
					c2total += ((c2 << C2_SHIFT) + ((1 << C2_SHIFT) >> 1)) * count;
				}
			}
		}

	cinfo->colormap[0][icolor] = (JSAMPLE) ((c0total + (total >> 1)) / total);
	cinfo->colormap[1][icolor] = (JSAMPLE) ((c1total + (total >> 1)) / total);
	cinfo->colormap[2][icolor] = (JSAMPLE) ((c2total + (total >> 1)) / total);
}


LOCAL void select_colors(j_decompress_ptr cinfo, int desired_colors)
/* Master routine for color selection */
{
	boxptr          boxlist;
	int             numboxes;
	int             i;

	/* Allocate workspace for box list */
	boxlist = (boxptr) (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, desired_colors * SIZEOF(box));
	/* Initialize one box containing whole space */
	numboxes = 1;
	boxlist[0].c0min = 0;
	boxlist[0].c0max = MAXJSAMPLE >> C0_SHIFT;
	boxlist[0].c1min = 0;
	boxlist[0].c1max = MAXJSAMPLE >> C1_SHIFT;
	boxlist[0].c2min = 0;
	boxlist[0].c2max = MAXJSAMPLE >> C2_SHIFT;
	/* Shrink it to actually-used volume and set its statistics */
	update_box(cinfo, &boxlist[0]);
	/* Perform median-cut to produce final box list */
	numboxes = median_cut(cinfo, boxlist, numboxes, desired_colors);
	/* Compute the representative color for each box, fill colormap */
	for(i = 0; i < numboxes; i++)
		compute_color(cinfo, &boxlist[i], i);
	cinfo->actual_number_of_colors = numboxes;
	TRACEMS1(cinfo, 1, JTRC_QUANT_SELECTED, numboxes);
}


/*
 * These routines are concerned with the time-critical task of mapping input
 * colors to the nearest color in the selected colormap.
 *
 * We re-use the histogram space as an "inverse color map", essentially a
 * cache for the results of nearest-color searches.  All colors within a
 * histogram cell will be mapped to the same colormap entry, namely the one
 * closest to the cell's center.  This may not be quite the closest entry to
 * the actual input color, but it's almost as good.  A zero in the cache
 * indicates we haven't found the nearest color for that cell yet; the array
 * is cleared to zeroes before starting the mapping pass.  When we find the
 * nearest color for a cell, its colormap index plus one is recorded in the
 * cache for future use.  The pass2 scanning routines call fill_inverse_cmap
 * when they need to use an unfilled entry in the cache.
 *
 * Our method of efficiently finding nearest colors is based on the "locally
 * sorted search" idea described by Heckbert and on the incremental distance
 * calculation described by Spencer W. Thomas in chapter III.1 of Graphics
 * Gems II (James Arvo, ed.  Academic Press, 1991).  Thomas points out that
 * the distances from a given colormap entry to each cell of the histogram can
 * be computed quickly using an incremental method: the differences between
 * distances to adjacent cells themselves differ by a constant.  This allows a
 * fairly fast implementation of the "brute force" approach of computing the
 * distance from every colormap entry to every histogram cell.  Unfortunately,
 * it needs a work array to hold the best-distance-so-far for each histogram
 * cell (because the inner loop has to be over cells, not colormap entries).
 * The work array elements have to be INT32s, so the work array would need
 * 256Kb at our recommended precision.  This is not feasible in DOS machines.
 *
 * To get around these problems, we apply Thomas' method to compute the
 * nearest colors for only the cells within a small subbox of the histogram.
 * The work array need be only as big as the subbox, so the memory usage
 * problem is solved.  Furthermore, we need not fill subboxes that are never
 * referenced in pass2; many images use only part of the color gamut, so a
 * fair amount of work is saved.  An additional advantage of this
 * approach is that we can apply Heckbert's locality criterion to quickly
 * eliminate colormap entries that are far away from the subbox; typically
 * three-fourths of the colormap entries are rejected by Heckbert's criterion,
 * and we need not compute their distances to individual cells in the subbox.
 * The speed of this approach is heavily influenced by the subbox size: too
 * small means too much overhead, too big loses because Heckbert's criterion
 * can't eliminate as many colormap entries.  Empirically the best subbox
 * size seems to be about 1/512th of the histogram (1/8th in each direction).
 *
 * Thomas' article also describes a refined method which is asymptotically
 * faster than the brute-force method, but it is also far more complex and
 * cannot efficiently be applied to small subboxes.  It is therefore not
 * useful for programs intended to be portable to DOS machines.  On machines
 * with plenty of memory, filling the whole histogram in one shot with Thomas'
 * refined method might be faster than the present code --- but then again,
 * it might not be any faster, and it's certainly more complicated.
 */


/* log2(histogram cells in update box) for each axis; this can be adjusted */
#define BOX_C0_LOG  (HIST_C0_BITS-3)
#define BOX_C1_LOG  (HIST_C1_BITS-3)
#define BOX_C2_LOG  (HIST_C2_BITS-3)

#define BOX_C0_ELEMS  (1<<BOX_C0_LOG)	/* # of hist cells in update box */
#define BOX_C1_ELEMS  (1<<BOX_C1_LOG)
#define BOX_C2_ELEMS  (1<<BOX_C2_LOG)

#define BOX_C0_SHIFT  (C0_SHIFT + BOX_C0_LOG)
#define BOX_C1_SHIFT  (C1_SHIFT + BOX_C1_LOG)
#define BOX_C2_SHIFT  (C2_SHIFT + BOX_C2_LOG)


/*
 * The next three routines implement inverse colormap filling.  They could
 * all be folded into one big routine, but splitting them up this way saves
 * some stack space (the mindist[] and bestdist[] arrays need not coexist)
 * and may allow some compilers to produce better code by registerizing more
 * inner-loop variables.
 */

LOCAL int find_nearby_colors(j_decompress_ptr cinfo, int minc0, int minc1, int minc2, JSAMPLE colorlist[])
/* Locate the colormap entries close enough to an update box to be candidates
 * for the nearest entry to some cell(s) in the update box.  The update box
 * is specified by the center coordinates of its first cell.  The number of
 * candidate colormap entries is returned, and their colormap indexes are
 * placed in colorlist[].
 * This routine uses Heckbert's "locally sorted search" criterion to select
 * the colors that need further consideration.
 */
{
	int             numcolors = cinfo->actual_number_of_colors;
	int             maxc0, maxc1, maxc2;
	int             centerc0, centerc1, centerc2;
	int             i, x, ncolors;
	INT32           minmaxdist, min_dist, max_dist, tdist;
	INT32           mindist[MAXNUMCOLORS];	/* min distance to colormap entry i */

	/* Compute true coordinates of update box's upper corner and center.
	 * Actually we compute the coordinates of the center of the upper-corner
	 * histogram cell, which are the upper bounds of the volume we care about.
	 * Note that since ">>" rounds down, the "center" values may be closer to
	 * min than to max; hence comparisons to them must be "<=", not "<".
	 */
	maxc0 = minc0 + ((1 << BOX_C0_SHIFT) - (1 << C0_SHIFT));
	centerc0 = (minc0 + maxc0) >> 1;
	maxc1 = minc1 + ((1 << BOX_C1_SHIFT) - (1 << C1_SHIFT));
	centerc1 = (minc1 + maxc1) >> 1;
	maxc2 = minc2 + ((1 << BOX_C2_SHIFT) - (1 << C2_SHIFT));
	centerc2 = (minc2 + maxc2) >> 1;

	/* For each color in colormap, find:
	 *  1. its minimum squared-distance to any point in the update box
	 *     (zero if color is within update box);
	 *  2. its maximum squared-distance to any point in the update box.
	 * Both of these can be found by considering only the corners of the box.
	 * We save the minimum distance for each color in mindist[];
	 * only the smallest maximum distance is of interest.
	 */
	minmaxdist = 0x7FFFFFFFL;

	for(i = 0; i < numcolors; i++)
	{
		/* We compute the squared-c0-distance term, then add in the other two. */
		x = GETJSAMPLE(cinfo->colormap[0][i]);
		if(x < minc0)
		{
			tdist = (x - minc0) * C0_SCALE;
			min_dist = tdist * tdist;
			tdist = (x - maxc0) * C0_SCALE;
			max_dist = tdist * tdist;
		}
		else if(x > maxc0)
		{
			tdist = (x - maxc0) * C0_SCALE;
			min_dist = tdist * tdist;
			tdist = (x - minc0) * C0_SCALE;
			max_dist = tdist * tdist;
		}
		else
		{
			/* within cell range so no contribution to min_dist */
			min_dist = 0;
			if(x <= centerc0)
			{
				tdist = (x - maxc0) * C0_SCALE;
				max_dist = tdist * tdist;
			}
			else
			{
				tdist = (x - minc0) * C0_SCALE;
				max_dist = tdist * tdist;
			}
		}

		x = GETJSAMPLE(cinfo->colormap[1][i]);
		if(x < minc1)
		{
			tdist = (x - minc1) * C1_SCALE;
			min_dist += tdist * tdist;
			tdist = (x - maxc1) * C1_SCALE;
			max_dist += tdist * tdist;
		}
		else if(x > maxc1)
		{
			tdist = (x - maxc1) * C1_SCALE;
			min_dist += tdist * tdist;
			tdist = (x - minc1) * C1_SCALE;
			max_dist += tdist * tdist;
		}
		else
		{
			/* within cell range so no contribution to min_dist */
			if(x <= centerc1)
			{
				tdist = (x - maxc1) * C1_SCALE;
				max_dist += tdist * tdist;
			}
			else
			{
				tdist = (x - minc1) * C1_SCALE;
				max_dist += tdist * tdist;
			}
		}

		x = GETJSAMPLE(cinfo->colormap[2][i]);
		if(x < minc2)
		{
			tdist = (x - minc2) * C2_SCALE;
			min_dist += tdist * tdist;
			tdist = (x - maxc2) * C2_SCALE;
			max_dist += tdist * tdist;
		}
		else if(x > maxc2)
		{
			tdist = (x - maxc2) * C2_SCALE;
			min_dist += tdist * tdist;
			tdist = (x - minc2) * C2_SCALE;
			max_dist += tdist * tdist;
		}
		else
		{
			/* within cell range so no contribution to min_dist */
			if(x <= centerc2)
			{
				tdist = (x - maxc2) * C2_SCALE;
				max_dist += tdist * tdist;
			}
			else
			{
				tdist = (x - minc2) * C2_SCALE;
				max_dist += tdist * tdist;
			}
		}

		mindist[i] = min_dist;	/* save away the results */
		if(max_dist < minmaxdist)
			minmaxdist = max_dist;
	}

	/* Now we know that no cell in the update box is more than minmaxdist
	 * away from some colormap entry.  Therefore, only colors that are
	 * within minmaxdist of some part of the box need be considered.
	 */
	ncolors = 0;
	for(i = 0; i < numcolors; i++)
	{
		if(mindist[i] <= minmaxdist)
			colorlist[ncolors++] = (JSAMPLE) i;
	}
	return ncolors;
}


LOCAL void
find_best_colors(j_decompress_ptr cinfo, int minc0, int minc1, int minc2, int numcolors, JSAMPLE colorlist[], JSAMPLE bestcolor[])
/* Find the closest colormap entry for each cell in the update box,
 * given the list of candidate colors prepared by find_nearby_colors.
 * Return the indexes of the closest entries in the bestcolor[] array.
 * This routine uses Thomas' incremental distance calculation method to
 * find the distance from a colormap entry to successive cells in the box.
 */
{
	int             ic0, ic1, ic2;
	int             i, icolor;
	register INT32 *bptr;		/* pointer into bestdist[] array */
	JSAMPLE        *cptr;		/* pointer into bestcolor[] array */
	INT32           dist0, dist1;	/* initial distance values */
	register INT32  dist2;		/* current distance in inner loop */
	INT32           xx0, xx1;	/* distance increments */
	register INT32  xx2;
	INT32           inc0, inc1, inc2;	/* initial values for increments */

	/* This array holds the distance to the nearest-so-far color for each cell */
	INT32           bestdist[BOX_C0_ELEMS * BOX_C1_ELEMS * BOX_C2_ELEMS];

	/* Initialize best-distance for each cell of the update box */
	bptr = bestdist;
	for(i = BOX_C0_ELEMS * BOX_C1_ELEMS * BOX_C2_ELEMS - 1; i >= 0; i--)
		*bptr++ = 0x7FFFFFFFL;

	/* For each color selected by find_nearby_colors,
	 * compute its distance to the center of each cell in the box.
	 * If that's less than best-so-far, update best distance and color number.
	 */

	/* Nominal steps between cell centers ("x" in Thomas article) */
#define STEP_C0  ((1 << C0_SHIFT) * C0_SCALE)
#define STEP_C1  ((1 << C1_SHIFT) * C1_SCALE)
#define STEP_C2  ((1 << C2_SHIFT) * C2_SCALE)

	for(i = 0; i < numcolors; i++)
	{
		icolor = GETJSAMPLE(colorlist[i]);
		/* Compute (square of) distance from minc0/c1/c2 to this color */
		inc0 = (minc0 - GETJSAMPLE(cinfo->colormap[0][icolor])) * C0_SCALE;
		dist0 = inc0 * inc0;
		inc1 = (minc1 - GETJSAMPLE(cinfo->colormap[1][icolor])) * C1_SCALE;
		dist0 += inc1 * inc1;
		inc2 = (minc2 - GETJSAMPLE(cinfo->colormap[2][icolor])) * C2_SCALE;
		dist0 += inc2 * inc2;
		/* Form the initial difference increments */
		inc0 = inc0 * (2 * STEP_C0) + STEP_C0 * STEP_C0;
		inc1 = inc1 * (2 * STEP_C1) + STEP_C1 * STEP_C1;
		inc2 = inc2 * (2 * STEP_C2) + STEP_C2 * STEP_C2;
		/* Now loop over all cells in box, updating distance per Thomas method */
		bptr = bestdist;
		cptr = bestcolor;
		xx0 = inc0;
		for(ic0 = BOX_C0_ELEMS - 1; ic0 >= 0; ic0--)
		{
			dist1 = dist0;
			xx1 = inc1;
			for(ic1 = BOX_C1_ELEMS - 1; ic1 >= 0; ic1--)
			{
				dist2 = dist1;
				xx2 = inc2;
				for(ic2 = BOX_C2_ELEMS - 1; ic2 >= 0; ic2--)
				{
					if(dist2 < *bptr)
					{
						*bptr = dist2;
						*cptr = (JSAMPLE) icolor;
					}
					dist2 += xx2;
					xx2 += 2 * STEP_C2 * STEP_C2;
					bptr++;
					cptr++;
				}
				dist1 += xx1;
				xx1 += 2 * STEP_C1 * STEP_C1;
			}
			dist0 += xx0;
			xx0 += 2 * STEP_C0 * STEP_C0;
		}
	}
}


LOCAL void fill_inverse_cmap(j_decompress_ptr cinfo, int c0, int c1, int c2)
/* Fill the inverse-colormap entries in the update box that contains */
/* histogram cell c0/c1/c2.  (Only that one cell MUST be filled, but */
/* we can fill as many others as we wish.) */
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	hist3d          histogram = cquantize->histogram;
	int             minc0, minc1, minc2;	/* lower left corner of update box */
	int             ic0, ic1, ic2;
	register JSAMPLE *cptr;		/* pointer into bestcolor[] array */
	register histptr cachep;	/* pointer into main cache array */

	/* This array lists the candidate colormap indexes. */
	JSAMPLE         colorlist[MAXNUMCOLORS];
	int             numcolors;	/* number of candidate colors */

	/* This array holds the actually closest colormap index for each cell. */
	JSAMPLE         bestcolor[BOX_C0_ELEMS * BOX_C1_ELEMS * BOX_C2_ELEMS];

	/* Convert cell coordinates to update box ID */
	c0 >>= BOX_C0_LOG;
	c1 >>= BOX_C1_LOG;
	c2 >>= BOX_C2_LOG;

	/* Compute true coordinates of update box's origin corner.
	 * Actually we compute the coordinates of the center of the corner
	 * histogram cell, which are the lower bounds of the volume we care about.
	 */
	minc0 = (c0 << BOX_C0_SHIFT) + ((1 << C0_SHIFT) >> 1);
	minc1 = (c1 << BOX_C1_SHIFT) + ((1 << C1_SHIFT) >> 1);
	minc2 = (c2 << BOX_C2_SHIFT) + ((1 << C2_SHIFT) >> 1);

	/* Determine which colormap entries are close enough to be candidates
	 * for the nearest entry to some cell in the update box.
	 */
	numcolors = find_nearby_colors(cinfo, minc0, minc1, minc2, colorlist);

	/* Determine the actually nearest colors. */
	find_best_colors(cinfo, minc0, minc1, minc2, numcolors, colorlist, bestcolor);

	/* Save the best color numbers (plus 1) in the main cache array */
	c0 <<= BOX_C0_LOG;			/* convert ID back to base cell indexes */
	c1 <<= BOX_C1_LOG;
	c2 <<= BOX_C2_LOG;
	cptr = bestcolor;
	for(ic0 = 0; ic0 < BOX_C0_ELEMS; ic0++)
	{
		for(ic1 = 0; ic1 < BOX_C1_ELEMS; ic1++)
		{
			cachep = &histogram[c0 + ic0][c1 + ic1][c2];
			for(ic2 = 0; ic2 < BOX_C2_ELEMS; ic2++)
			{
				*cachep++ = (histcell) (GETJSAMPLE(*cptr++) + 1);
			}
		}
	}
}


/*
 * Map some rows of pixels to the output colormapped representation.
 */

METHODDEF void pass2_no_dither(j_decompress_ptr cinfo, JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
/* This version performs no dithering */
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	hist3d          histogram = cquantize->histogram;
	register JSAMPROW inptr, outptr;
	register histptr cachep;
	register int    c0, c1, c2;
	int             row;
	JDIMENSION      col;
	JDIMENSION      width = cinfo->output_width;

	for(row = 0; row < num_rows; row++)
	{
		inptr = input_buf[row];
		outptr = output_buf[row];
		for(col = width; col > 0; col--)
		{
			/* get pixel value and index into the cache */
			c0 = GETJSAMPLE(*inptr++) >> C0_SHIFT;
			c1 = GETJSAMPLE(*inptr++) >> C1_SHIFT;
			c2 = GETJSAMPLE(*inptr++) >> C2_SHIFT;
			cachep = &histogram[c0][c1][c2];
			/* If we have not seen this color before, find nearest colormap entry */
			/* and update the cache */
			if(*cachep == 0)
				fill_inverse_cmap(cinfo, c0, c1, c2);
			/* Now emit the colormap index for this cell */
			*outptr++ = (JSAMPLE) (*cachep - 1);
		}
	}
}


METHODDEF void pass2_fs_dither(j_decompress_ptr cinfo, JSAMPARRAY input_buf, JSAMPARRAY output_buf, int num_rows)
/* This version performs Floyd-Steinberg dithering */
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	hist3d          histogram = cquantize->histogram;
	register LOCFSERROR cur0, cur1, cur2;	/* current error or pixel value */
	LOCFSERROR      belowerr0, belowerr1, belowerr2;	/* error for pixel below cur */
	LOCFSERROR      bpreverr0, bpreverr1, bpreverr2;	/* error for below/prev col */
	register FSERRPTR errorptr;	/* => fserrors[] at column before current */
	JSAMPROW        inptr;		/* => current input pixel */
	JSAMPROW        outptr;		/* => current output pixel */
	histptr         cachep;
	int             dir;		/* +1 or -1 depending on direction */
	int             dir3;		/* 3*dir, for advancing inptr & errorptr */
	int             row;
	JDIMENSION      col;
	JDIMENSION      width = cinfo->output_width;
	JSAMPLE        *range_limit = cinfo->sample_range_limit;
	int            *error_limit = cquantize->error_limiter;
	JSAMPROW        colormap0 = cinfo->colormap[0];
	JSAMPROW        colormap1 = cinfo->colormap[1];
	JSAMPROW        colormap2 = cinfo->colormap[2];

	SHIFT_TEMPS for(row = 0; row < num_rows; row++)
	{
		inptr = input_buf[row];
		outptr = output_buf[row];
		if(cquantize->on_odd_row)
		{
			/* work right to left in this row */
			inptr += (width - 1) * 3;	/* so point to rightmost pixel */
			outptr += width - 1;
			dir = -1;
			dir3 = -3;
			errorptr = cquantize->fserrors + (width + 1) * 3;	/* => entry after last column */
			cquantize->on_odd_row = FALSE;	/* flip for next time */
		}
		else
		{
			/* work left to right in this row */
			dir = 1;
			dir3 = 3;
			errorptr = cquantize->fserrors;	/* => entry before first real column */
			cquantize->on_odd_row = TRUE;	/* flip for next time */
		}
		/* Preset error values: no error propagated to first pixel from left */
		cur0 = cur1 = cur2 = 0;
		/* and no error propagated to row below yet */
		belowerr0 = belowerr1 = belowerr2 = 0;
		bpreverr0 = bpreverr1 = bpreverr2 = 0;

		for(col = width; col > 0; col--)
		{
			/* curN holds the error propagated from the previous pixel on the
			 * current line.  Add the error propagated from the previous line
			 * to form the complete error correction term for this pixel, and
			 * round the error term (which is expressed * 16) to an integer.
			 * RIGHT_SHIFT rounds towards minus infinity, so adding 8 is correct
			 * for either sign of the error value.
			 * Note: errorptr points to *previous* column's array entry.
			 */
			cur0 = RIGHT_SHIFT(cur0 + errorptr[dir3 + 0] + 8, 4);
			cur1 = RIGHT_SHIFT(cur1 + errorptr[dir3 + 1] + 8, 4);
			cur2 = RIGHT_SHIFT(cur2 + errorptr[dir3 + 2] + 8, 4);
			/* Limit the error using transfer function set by init_error_limit.
			 * See comments with init_error_limit for rationale.
			 */
			cur0 = error_limit[cur0];
			cur1 = error_limit[cur1];
			cur2 = error_limit[cur2];
			/* Form pixel value + error, and range-limit to 0..MAXJSAMPLE.
			 * The maximum error is +- MAXJSAMPLE (or less with error limiting);
			 * this sets the required size of the range_limit array.
			 */
			cur0 += GETJSAMPLE(inptr[0]);
			cur1 += GETJSAMPLE(inptr[1]);
			cur2 += GETJSAMPLE(inptr[2]);
			cur0 = GETJSAMPLE(range_limit[cur0]);
			cur1 = GETJSAMPLE(range_limit[cur1]);
			cur2 = GETJSAMPLE(range_limit[cur2]);
			/* Index into the cache with adjusted pixel value */
			cachep = &histogram[cur0 >> C0_SHIFT][cur1 >> C1_SHIFT][cur2 >> C2_SHIFT];
			/* If we have not seen this color before, find nearest colormap */
			/* entry and update the cache */
			if(*cachep == 0)
				fill_inverse_cmap(cinfo, cur0 >> C0_SHIFT, cur1 >> C1_SHIFT, cur2 >> C2_SHIFT);
			/* Now emit the colormap index for this cell */
			{
				register int    pixcode = *cachep - 1;

				*outptr = (JSAMPLE) pixcode;
				/* Compute representation error for this pixel */
				cur0 -= GETJSAMPLE(colormap0[pixcode]);
				cur1 -= GETJSAMPLE(colormap1[pixcode]);
				cur2 -= GETJSAMPLE(colormap2[pixcode]);
			}
			/* Compute error fractions to be propagated to adjacent pixels.
			 * Add these into the running sums, and simultaneously shift the
			 * next-line error sums left by 1 column.
			 */
			{
				register LOCFSERROR bnexterr, delta;

				bnexterr = cur0;	/* Process component 0 */
				delta = cur0 * 2;
				cur0 += delta;	/* form error * 3 */
				errorptr[0] = (FSERROR) (bpreverr0 + cur0);
				cur0 += delta;	/* form error * 5 */
				bpreverr0 = belowerr0 + cur0;
				belowerr0 = bnexterr;
				cur0 += delta;	/* form error * 7 */
				bnexterr = cur1;	/* Process component 1 */
				delta = cur1 * 2;
				cur1 += delta;	/* form error * 3 */
				errorptr[1] = (FSERROR) (bpreverr1 + cur1);
				cur1 += delta;	/* form error * 5 */
				bpreverr1 = belowerr1 + cur1;
				belowerr1 = bnexterr;
				cur1 += delta;	/* form error * 7 */
				bnexterr = cur2;	/* Process component 2 */
				delta = cur2 * 2;
				cur2 += delta;	/* form error * 3 */
				errorptr[2] = (FSERROR) (bpreverr2 + cur2);
				cur2 += delta;	/* form error * 5 */
				bpreverr2 = belowerr2 + cur2;
				belowerr2 = bnexterr;
				cur2 += delta;	/* form error * 7 */
			}
			/* At this point curN contains the 7/16 error value to be propagated
			 * to the next pixel on the current line, and all the errors for the
			 * next line have been shifted over.  We are therefore ready to move on.
			 */
			inptr += dir3;		/* Advance pixel pointers to next column */
			outptr += dir;
			errorptr += dir3;	/* advance errorptr to current column */
		}
		/* Post-loop cleanup: we must unload the final error values into the
		 * final fserrors[] entry.  Note we need not unload belowerrN because
		 * it is for the dummy column before or after the actual array.
		 */
		errorptr[0] = (FSERROR) bpreverr0;	/* unload prev errs into array */
		errorptr[1] = (FSERROR) bpreverr1;
		errorptr[2] = (FSERROR) bpreverr2;
	}
}


/*
 * Initialize the error-limiting transfer function (lookup table).
 * The raw F-S error computation can potentially compute error values of up to
 * +- MAXJSAMPLE.  But we want the maximum correction applied to a pixel to be
 * much less, otherwise obviously wrong pixels will be created.  (Typical
 * effects include weird fringes at color-area boundaries, isolated bright
 * pixels in a dark area, etc.)  The standard advice for avoiding this problem
 * is to ensure that the "corners" of the color cube are allocated as output
 * colors; then repeated errors in the same direction cannot cause cascading
 * error buildup.  However, that only prevents the error from getting
 * completely out of hand; Aaron Giles reports that error limiting improves
 * the results even with corner colors allocated.
 * A simple clamping of the error values to about +- MAXJSAMPLE/8 works pretty
 * well, but the smoother transfer function used below is even better.  Thanks
 * to Aaron Giles for this idea.
 */

LOCAL void init_error_limit(j_decompress_ptr cinfo)
/* Allocate and fill in the error_limiter table */
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	int            *table;
	int             in, out;

	table = (int *)(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, (MAXJSAMPLE * 2 + 1) * SIZEOF(int));

	table += MAXJSAMPLE;		/* so can index -MAXJSAMPLE .. +MAXJSAMPLE */
	cquantize->error_limiter = table;

#define STEPSIZE ((MAXJSAMPLE+1)/16)
	/* Map errors 1:1 up to +- MAXJSAMPLE/16 */
	out = 0;
	for(in = 0; in < STEPSIZE; in++, out++)
	{
		table[in] = out;
		table[-in] = -out;
	}
	/* Map errors 1:2 up to +- 3*MAXJSAMPLE/16 */
	for(; in < STEPSIZE * 3; in++, out += (in & 1) ? 0 : 1)
	{
		table[in] = out;
		table[-in] = -out;
	}
	/* Clamp the rest to final out value (which is (MAXJSAMPLE+1)/8) */
	for(; in <= MAXJSAMPLE; in++)
	{
		table[in] = out;
		table[-in] = -out;
	}
#undef STEPSIZE
}


/*
 * Finish up at the end of each pass.
 */

METHODDEF void finish_pass1(j_decompress_ptr cinfo)
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;

	/* Select the representative colors and fill in cinfo->colormap */
	cinfo->colormap = cquantize->sv_colormap;
	select_colors(cinfo, cquantize->desired);
	/* Force next pass to zero the color index table */
	cquantize->needs_zeroed = TRUE;
}


METHODDEF void finish_pass2(j_decompress_ptr cinfo)
{
	/* no work */
}


/*
 * Initialize for each processing pass.
 */

METHODDEF void start_pass_2_quant(j_decompress_ptr cinfo, boolean is_pre_scan)
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;
	hist3d          histogram = cquantize->histogram;
	int             i;

	/* Only F-S dithering or no dithering is supported. */
	/* If user asks for ordered dither, give him F-S. */
	if(cinfo->dither_mode != JDITHER_NONE)
		cinfo->dither_mode = JDITHER_FS;

	if(is_pre_scan)
	{
		/* Set up method pointers */
		cquantize->pub.color_quantize = prescan_quantize;
		cquantize->pub.finish_pass = finish_pass1;
		cquantize->needs_zeroed = TRUE;	/* Always zero histogram */
	}
	else
	{
		/* Set up method pointers */
		if(cinfo->dither_mode == JDITHER_FS)
			cquantize->pub.color_quantize = pass2_fs_dither;
		else
			cquantize->pub.color_quantize = pass2_no_dither;
		cquantize->pub.finish_pass = finish_pass2;

		/* Make sure color count is acceptable */
		i = cinfo->actual_number_of_colors;
		if(i < 1)
			ERREXIT1(cinfo, JERR_QUANT_FEW_COLORS, 1);
		if(i > MAXNUMCOLORS)
			ERREXIT1(cinfo, JERR_QUANT_MANY_COLORS, MAXNUMCOLORS);

		if(cinfo->dither_mode == JDITHER_FS)
		{
			size_t          arraysize = (size_t) ((cinfo->output_width + 2) * (3 * SIZEOF(FSERROR)));

			/* Allocate Floyd-Steinberg workspace if we didn't already. */
			if(cquantize->fserrors == NULL)
				cquantize->fserrors = (FSERRPTR) (*cinfo->mem->alloc_large) ((j_common_ptr) cinfo, JPOOL_IMAGE, arraysize);
			/* Initialize the propagated errors to zero. */
			jzero_far((void FAR *)cquantize->fserrors, arraysize);
			/* Make the error-limit table if we didn't already. */
			if(cquantize->error_limiter == NULL)
				init_error_limit(cinfo);
			cquantize->on_odd_row = FALSE;
		}

	}
	/* Zero the histogram or inverse color map, if necessary */
	if(cquantize->needs_zeroed)
	{
		for(i = 0; i < HIST_C0_ELEMS; i++)
		{
			jzero_far((void FAR *)histogram[i], HIST_C1_ELEMS * HIST_C2_ELEMS * SIZEOF(histcell));
		}
		cquantize->needs_zeroed = FALSE;
	}
}


/*
 * Switch to a new external colormap between output passes.
 */

METHODDEF void new_color_map_2_quant(j_decompress_ptr cinfo)
{
	my_cquantize_ptr cquantize = (my_cquantize_ptr) cinfo->cquantize;

	/* Reset the inverse color map */
	cquantize->needs_zeroed = TRUE;
}


/*
 * Module initialization routine for 2-pass color quantization.
 */

GLOBAL void jinit_2pass_quantizer(j_decompress_ptr cinfo)
{
	my_cquantize_ptr cquantize;
	int             i;

	cquantize = (my_cquantize_ptr) (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, SIZEOF(my_cquantizer));
	cinfo->cquantize = (struct jpeg_color_quantizer *)cquantize;
	cquantize->pub.start_pass = start_pass_2_quant;
	cquantize->pub.new_color_map = new_color_map_2_quant;
	cquantize->fserrors = NULL;	/* flag optional arrays not allocated */
	cquantize->error_limiter = NULL;

	/* Make sure jdmaster didn't give me a case I can't handle */
	if(cinfo->out_color_components != 3)
		ERREXIT(cinfo, JERR_NOTIMPL);

	/* Allocate the histogram/inverse colormap storage */
	cquantize->histogram = (hist3d) (*cinfo->mem->alloc_small)
		((j_common_ptr) cinfo, JPOOL_IMAGE, HIST_C0_ELEMS * SIZEOF(hist2d));
	for(i = 0; i < HIST_C0_ELEMS; i++)
	{
		cquantize->histogram[i] = (hist2d) (*cinfo->mem->alloc_large)
			((j_common_ptr) cinfo, JPOOL_IMAGE, HIST_C1_ELEMS * HIST_C2_ELEMS * SIZEOF(histcell));
	}
	cquantize->needs_zeroed = TRUE;	/* histogram is garbage now */

	/* Allocate storage for the completed colormap, if required.
	 * We do this now since it is FAR storage and may affect
	 * the memory manager's space calculations.
	 */
	if(cinfo->enable_2pass_quant)
	{
		/* Make sure color count is acceptable */
		int             desired = cinfo->desired_number_of_colors;

		/* Lower bound on # of colors ... somewhat arbitrary as long as > 0 */
		if(desired < 8)
			ERREXIT1(cinfo, JERR_QUANT_FEW_COLORS, 8);
		/* Make sure colormap indexes can be represented by JSAMPLEs */
		if(desired > MAXNUMCOLORS)
			ERREXIT1(cinfo, JERR_QUANT_MANY_COLORS, MAXNUMCOLORS);
		cquantize->sv_colormap = (*cinfo->mem->alloc_sarray)
			((j_common_ptr) cinfo, JPOOL_IMAGE, (JDIMENSION) desired, (JDIMENSION) 3);
		cquantize->desired = desired;
	}
	else
		cquantize->sv_colormap = NULL;

	/* Only F-S dithering or no dithering is supported. */
	/* If user asks for ordered dither, give him F-S. */
	if(cinfo->dither_mode != JDITHER_NONE)
		cinfo->dither_mode = JDITHER_FS;

	/* Allocate Floyd-Steinberg workspace if necessary.
	 * This isn't really needed until pass 2, but again it is FAR storage.
	 * Although we will cope with a later change in dither_mode,
	 * we do not promise to honor max_memory_to_use if dither_mode changes.
	 */
	if(cinfo->dither_mode == JDITHER_FS)
	{
		cquantize->fserrors = (FSERRPTR) (*cinfo->mem->alloc_large)
			((j_common_ptr) cinfo, JPOOL_IMAGE, (size_t) ((cinfo->output_width + 2) * (3 * SIZEOF(FSERROR))));
		/* Might as well create the error-limiting table too. */
		init_error_limit(cinfo);
	}
}

#endif							/* QUANT_2PASS_SUPPORTED */
